A known structure for receiving surgical instruments is disclosed in U.S. Pat. No. 6,187,000. U.S. Pat. No. 6,187,000 discloses a cannula having an expandable portion. The expandable portion has a slot and a guide member disposed in the slot. The guide member is movable from a first terminal end of the slot to a second terminal end of the slot to enable the cross-sectional area of a passage in the cannula to increase.